It is common to configure a disk drive with multiple disks that rotate on a common spindle. A disk drive of this type has a read/write assembly in which read/write heads are mounted on actuator arms that are fixed to a common shaft or housing which is rotated to position the heads on respective circular data tracks on the disk surfaces.
The multiple-disk arrangement provides two important advantages. The first of these is compactness and relatively low cost. The second advantage relates to the speed of data access. Specifically, the data in consecutive logical locations is physically located in "cylinders", i.e. series of tracks having the same radii but disposed on a succession of disk surfaces. Since the read/write heads move in unison, they are, at any given time, located over tracks in the same cylinder. Thus, when data occupying more than one track in a cylinder is to be accessed, the data in the first track is retrieved by means of a read/write head positioned on that track and the system then switches to another head which is already positioned on the track carrying the next portion of the data to be retrieved.
This should be contrasted with a single disk surface, where the head must be repositioned radially whenever data from another track is to be accessed. This repositioning or "track seeking" operation is relatively slow in the context of the data transfer rates involved in disk storage and retrieval. Thus the reduction in the number of such operations provided by multiple-disk arrangements materially improves the overall speed of operation.
The evolution of disk drives has been characterized by continual increases in the density of data stored on the disks, in terms of both the number of bits per track and the spacing of the tracks. Unfortunately this can significantly increase the time required to switch from one read/write head to another. Ideally, the heads are simultaneously in position on the respective tracks in the same cylinder as described above. However, temperature changes cause small variations in the radial positions of the various read/write heads. With a high track-density and the concomitant requirement for highly precise radial positioning of the read/write heads, this variation can cause a head to be out of position radially even though another head is centered on a data track. Accordingly, even when the system switches from one head to another for operation in the same cylinder, it must wait for the head-positioning servo system to move the head assembly so as to center the second head on a track.